In the two weeks she was alive in 2008 — only females feed on blood — the mosquito encountered a bird that turned her into an agent of death.

The bird was a carrier of Flavivirus, commonly known as West Nile virus. When the mosquito, most likely Culex pipens, fed on the bird, the virus jumped to the mosquito, where it began to replicate.

After feeding on the bird, she landed on a 76-year-old man, jamming her needle-like mouthpart into his skin, spitting an anti-coagulate into him and sucking his blood.

The mosquito's saliva was packed with the virus, which allowed it to travel from the mosquito to the man's blood stream, where it caught a ride to his lymph nodes, the virus' preferred home in humans.

A month later, he was dead.

This man's death in 2008 appeared, at least for the moment, to mark a turning point in the spread of West Nile.

At the state Department of Environmental Protection's laboratories in Dauphin County, researchers track the virus through the evidence it leaves behind — dead birds and mosquito samples. For years, the infection rates in humans had been declining, and for a few years, the Philadelphia man's death was the last attributed to the virus in Pennsylvania.

But West Nile is a dynamic, tough nut to crack — give it an inch, and it'll take a mile. Last year, the virus came roaring back with 60 confirmed human cases and four deaths statewide, according to state officials.

A deadly outlier

View full sizeThese frozen mosquito samples are in the process of being sorted for testing at the state Department of Environmental Protection labs in Harrisburg, where they test mosquito samples for West Nile virus.
Daniel Zampogna, PennLive

At the state Department of Health, Dr. James Rankin has been studying — and fighting — West Nile, since it first appeared in the United States in 1999.

The virus is primarily deadly only to the elderly, or those with suppressed immune systems. While the mortality rates for the virus might not be as high as other viruses, “it is a significant disease,” he said.

“We have people dying, people getting sick,” Rankin said, adding that the long-term effects, which include fatigue and cognitive difficulties, can haunt a person their entire life. “It certainly is something we need to be cautious about.”

Rankin credits the state's efforts in education and mosquito control with the decline between 2003 and 2011.

But 2012 was an outlier, with a warm spring and summer that provided the perfect conditions for West Nile’s comeback.

During a warm spring, the mosquitos hatch early and prey on baby birds, the Flavivirus' preferred host. And a warm summer means more people are outdoors, creating more opportunities for mosquitoes to prey on humans.

“Step by step, inch by inch, you have the equivalent of a perfect mosquito season,” Rankin said.

Containment fails

View full sizePhil Hall, West Nile Virus coordinator with the Penn State Cooperative Extension in Lebanon County, sprays a granular pesticide to kill mosquito larvae in standing water in March 2010 at YMCA Camp Shand in South Lebanon Township.
DAN GLEITER, The Patriot-News

West Nile was first recognized in 1937 in Uganda, although scientists speculate that the virus could be more than 1,000 years old, according to science experts.

For years, it was confined to Africa and the Middle East, with occasional outbreaks in southern and eastern Europe in the 1980s and early 1990s.

In 1999, the virus arrived in Queens, N.Y., where in August of that year a physician noticed a pattern of brain inflammation among several of his patients.

Within days, other patients began cropping up and the case was forwarded to the Centers for Disease Control. The state of New York responded by initiating a control program, spraying insecticides to try and curb the virus-carrying mosquito population.

Attempts at containment failed.

Within a month of the initial physician's report, four people were dead and the virus had spread to the surrounding counties.

Meanwhile, health officials — including
zookeepers at the Bronx Zoo — began seeing an increasing number of dead birds
in the region.

Although the virus is spread between species by mosquitoes, dead birds are the true harbinger of West Nile.

While it will infect humans, we are dead-end hosts for the virus. Horses are too, and the virus has been the death of at least two killer whales. It is only in birds that the virus can survive and replicate long term.

While the virus thrives in birds, it also kills them, albeit slowly enough to allow itself to spread. In winter months, the virus hibernates in adult mosquitoes that live in barns, caves and root cellars.

Although the virus' lifecycle may appear tenuous, it also provides for rapid mobility. It entered Pennsylvania in 2000, and spread to Maryland, Florida, Georgia and Alabama in 2001.

By 2003, the virus had spread to most of the contiguous United States.

That year, West Nile mutated, and events aligned for a perfect storm in the nation and Pennsylvania and the number of human infections peaked.

In the commonwealth, more than 200 cases were reported. Nine people died.

“I don't know if we expected it to move that fast,” Rankin said. “But we certainly were concerned when it appeared in New York.”

Tracking a killer

View full sizeIntern Matt Samara, 22, of Harrisburg, carefully separates samples of frozen misquotes in the the state Department of Environmental Protection labs in Susquehanna Township, where they test mosquito samples for West Nile virus.
Daniel Zampogna, PennLive.com

Inside DEP's laboratory in Susquehanna Township, a medical centrifuge is whirling away. Inside, contained in a series of pipettes, pulverized mosquitoes are being slowly ripped apart by centrifugal force.

The larger chunks of mosquitoes are pulled toward the bottom of the tubes. The lighter bits — including possible West Nile virus cells — are held suspended in a solution. After whirling for a couple of hours, the solution is removed from the tubes and undergoes another sorting process, designed to break down cell walls.

In a tiny drop of liquid, the virus' walls are ruptured, allowing their contents, including the virus' RNA, or genetic material, to float free.

Yet another machine analyzes the RNA, comparing it against known markers. If the proper sequence of RNA is detected, the sample is flagged by the computer that monitors the reactions.

The DEP team performs these tests hundreds of times per day, on an estimated 25,000 mosquitoes. The mosquitoes are collected daily by field agents, and shipped to the central laboratory, where they are sorted — by hand — by species and geographic location.

If enough samples turn up positive, the state arranges for control spraying in the locale, using an insecticide derived from chrysanthemum flowers.

So far 2013 has been an odd year. The department's field agents have collected near-record numbers of mosquitoes — indicating a larger-than-normal population — but West Nile hasn't turned up as much as expected.

Instead the virus has lagged this year. The peak season is usually July through September, with early flags in June. This year, the lab didn't find any trace of the virus in June, and West Nile didn't start to really be discovered until late July.

“The [mosquito] population in July are the highest we've ever seen since the start of the program,” said Mike Hutchenson, who runs the DEP laboratory. Why that hasn't correlated with the virus is a mystery.

He speculates that the cooler spring may have retarded mosquito growth, allowing the bird population to mature ahead of the swarm. But it's hard, he said, to pin down why the virus spreads one year and not the next.

“West Nile is very dynamic,” he said. “There [are] a lot of factors that have to come together for it to be a problem.”

For now, West Nile appears once again to be on the decline. The natural immune responses of humans can protect them against the virus, and many people come into contact with it without even knowing it, Rankin said.

But the virus could mutate again, as it did in 2003, with the right sequence of weather events. And West Nile could come back.

At the DEP lab, Hutchenson handles a dead crow that has been sent in for analysis. His team will continue to test mosquito samples and birds that are forwarded their way.

But they can only monitor, and there's no way to know exactly what is happening with West Nile.

Like a forensic team tracking a serial killer, they can only measure the virus's passage by the clues it leaves behind.

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